A light emitting diode ("LED") is a photonic semiconductor device. Photonic semiconductor devices fall into three categories: devices that convert electrical energy into optical radiation (LED's and diode lasers), devices that detect optical signals (photodetectors), and devices that convert optical radiation into electrical energy (photovoltaic devices or solar cells).
Although all three of these categories or devices have useful applications, the LED may be the most commonly recognized because of its application to such a wide variety of products and applications such as scientific equipment, medical equipment, and perhaps most commonly, various consumer products in which LEDs form the light source for various signals, indicators, gauges, clocks, and many other familiar items.
Semiconductor sources such as LEDs are particularly desirable as light output devices in such items because of their generally long lifetime, their low power requirements, and their high reliability.
In spite of their widespread use, LEDs are to some extent functionally limited, because the color that an LED can produce is fundamentally limited by the nature of semiconductor materials in which the LED is formed. As well known to those of ordinary skill in this and related arts, the light produced by an LED is referred to as "electroluminescence" and represents the generation of light by an electric current passing through a material under an applied electric field. Any given material that produces electroluminescent light tends to do so over a relatively narrow range of wavelengths under given circumstances. Electroluminescence thus differs from thermal radiation or incandescence which generally have much broader spectral widths.
More fundamentally, an LED's luminescence is produced by basic quantum mechanical transitions between energy levels within the semiconductor material. Because the bands within a material depend both upon the material and its doping, the energy of the transition, and thus the color of the radiation it produces, is limited by the well known relationship (E=hv) between the energy (E) of a transition and the frequency (v) of the light it produces (h is Plank's constant). Blue light has a shorter wavelength (and thus a higher frequency) than the other colors in the visible spectrum, and thus must be produced from transitions that are greater in energy than those transitions which produce green, yellow, orange or red light.
More specifically, the entire visible spectrum runs from the violet at about 390 nanometers to the red at about 770 nanometers. In turn, the blue portion of the visible spectrum can be considered (somewhat arbitrarily) to extend between the wavelengths of about 425 and 480 nanometers. The wavelengths of 425 and 480 nanometers in turn represent energy transitions (also somewhat arbitrarily) of about 2.6 eV and about 2.9 eV. Accordingly, only materials with a bandgap of at least about 2.6 eV can produce blue light, even under the best of conditions.
As is further well recognized, blue is one of the primary colors, and thus any devices which hope to produce full color displays using LEDs need to incorporate blue in some fashion. Absent efficient blue LEDs, some other method such as filtering or shuttering must be used to produce a blue contribution to a display that otherwise lacks a blue LED source.
From another standpoint, blue light's shorter wavelength allows it to be used to store more information on optical memory devices (such as CD ROM) than can red or yellow light. In particular, a CD ROM of a given physical size can hold about eight times as much information using blue light than it could when using red light. Thus, the advantages for computer and other sorts of optical memories using blue light are quite attractive.
Candidate materials with sufficient bandgaps to produce blue light include silicon carbide, gallium nitride, other Group III nitrides, zinc sulfide, and zinc selenide. More common semiconductor materials such as silicon, gallium phosphide, or gallium arsenide are unsuitable for producing blue light because their bandgaps are on the order of 2.26 eV or less.
The last decade has seen a great deal of progress in both the basic and commercial development of blue light emitting diodes, including a number of contributions by the assignee of the present invention. These include U.S. Pat. Nos. 4,918,497; 4,966,862; 5,027,168; and 5,338,944.
Another candidate material for blue light emitting diodes is gallium nitride (GaN) and its analogous Group III (i.e. Group III of the periodic table) nitride compounds such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), indium aluminum nitride (InAlN) and in some circumstances indium aluminum gallium nitride (InAlGaN). These materials are particularly attractive because they offer direct energy transitions with bandgaps from between about 3.4 to about 6.2 eV at room temperature. As known to those familiar with LEDs and electronic transitions, a direct (or "vertical") transition occurs in a semiconductor when the valance band maxima and the conduction band minima have the same K value, which in turn means that crystal momentum is conserved during the transition so that the energy produced by the transition can go predominantly into the photon; i.e. to produce light rather than heat. When the conduction and valance band minima are not of the same K value, a phonon (i.e. an emission of vibrational energy) is required to conserve crystal momentum and the transition is called "indirect." The phonon's energy essentially reduces the energy of any resulting photon, thus reducing both the frequency and the intensity of the emitted light. A full discussion of the theory and operation of LEDs is given in Chapter 12 of Sze, Physics of Semiconductor Devices, Second Edition (1981), pp. 681ff.
From a more lay point of view, the direct transition characteristics of group III nitrides, including gallium nitride, offer the potential for brighter and more efficient emissions--and thus brighter and more efficient LEDS--than do the emissions from indirect materials such as silicon carbide, all other factors being generally equal.
Accordingly, much interest in the last decade has also focused on producing light emitting diodes in gallium nitride and the related group III nitrides.
Although gallium nitride offers a direct transition over a wide bandgap, and thus a theoretically greater brightness, the material presents a particular set of technical problems in manufacturing working devices. The primary problem is the lack of bulk single crystals of gallium nitride which in turn means that gallium nitride or other group III nitride devices must be formed as epitaxial layers on other materials. The most commonly used material to date has been sapphire (aluminum oxide, Al.sub.2 O.sub.3). Sapphire offers a reasonable crystal lattice match to Group III nitrides, thermal stability, and transparency, all of which are generally useful in producing a light emitting diode. Sapphire offers the disadvantage, however, of being unsuitable for conductivity doping. In turn, this means that the electric current that must be passed through an LED to generate the emission cannot be directed through the sapphire substrate. Thus other types of connections to the LED must be made. In general, LEDs with "vertical" geometry (i.e. using conductive substrates so that ohmic contacts can be placed at opposite ends of the device) are preferred for a number of reasons, including their easier manufacture than such "nonvertical" devices.
Accordingly, the assignee of the present invention has developed the use of silicon carbide substrates for gallium nitride and other Group III devices as a means of solving the conductivity problems of sapphire as a substrate. Because silicon carbide can be doped conductively, "vertical" LEDs can be formed on it; i.e. devices in which one contact can be made to the top of a device and the second contact to the bottom of the device, a structure which greatly facilitates the manufacture of the LED as well as the incorporation of the LED into circuits or combination devices or structures.
In spite of these theoretical advantages, a consistently reliable and predictable blue emission using a gallium nitride active layer has yet to be accomplished. For example, some workers have used silicon and zinc to co-dope or compensate indium gallium nitride (InGaN) but have interpreted their results to mean that gallium nitride standing alone is not suited as well as InGaN for an LED because InGaN gives a better band-to-band ("interband") transition for a light emitting diode.
Similarly, others have prepared heterojunctions from AlGaN and GaN, but never attempted or described compensated doping, and by all indications, used an intrinsic n-type gallium nitride and p-type AlGaN for the junction. Indeed, if some of the prior art structures are properly understood, doping them with a compensating acceptor such as zinc would have produced an insulating layer rather than a compensated one.